Behaviour of the bacterial community during the secondary treatment by activated sludge

Abstract

The treatment of wastewater is one of the most used biotechnological processes. Moreover, the secondary treatment by activated sludge is the most extended one. The flocs, the key unit of this system is made of organic and inorganic particles and has a concrete microbiological community in which bacteria play an important role. However, nowadays the legislation established the wastewater quality only by physiochemical parameters. Through this work we discovered how microbiological parameters are more sensitive and that flow cytometry is a useful tool for the automatization of the correct operation of a wastewater treatment plant. On the other hand, we could observe how the bacterial community changes after the secondary treatment, and so, even if the secondary effluent has fewer bacteria, its diversity increases. Another objective was to study the fate of faecal bacteria during the secondary treatment by activated sludge. To achieve this objective we used two genetically modified bacteria Escherichia coli and Serratia marcescens, both expressing the GFP protein. These bacteria were inoculated in the primary effluent of a laboratory-scale plant, that simulates the secondary treatment by activated sludge. The results showed a different pattern. Thus, the 30% of the inoculated cells that were not eliminated from the system were distributed differently, for E. coli, the 95% of these bacteria were adhered to the solid fractions while the S. marcescens cells were equally distributed between the aqueous and solid fractions. These results let us determine that the adhesion to sludge and predation by protozoa were the most important factors that affect the elimination of faecal bacteria during the secondary treatment by activated sludge. However, their importance, specially in the case of adsorption to sludge, largely varies between species. Moreover, and studying the results obtained for the two strains used during this study, we can conclude that it is necessary to review the real value of bacterial indicators to control the correct operation of the wastewater treatment, as they elimination patterns are not useful to predict the behaviour of other bacteria.